A. If he wants his own children, wtf has he been doing for the last 25 years??
B. If he's only recently decided he wants them, then surely he knows he needs a 10 years or younger woman. (And even 10 yrs would be pushing it, given the maternal & paternal ages).
Most women, in terms of egg quality, are effectively infertile by 45.
There are outliers of course, but the chances are extremely low and the risks are increasing every year.
C.. if he wants his own children, with no use of a sperm donor; he appears to be ignorant about the risks of older sperm too; like raised incidence of autism in kids if the father's over 50, raised risk of disorders like schizophrenia, the older the father.
Those are difficult enough for parents with no other kids to navigate, let alone someone with 3 kids already.
He needs someone considerably younger, and he'd still be taking some risks using his own sperm.
If the relationship with the mother breaks down, the typical set up is for the mother to be the resident parent and main carer, that's who usually shoulders the burden of additional needs.
He's in cloud cuckoo land, if he wants kids (at 50!!!) he should not have gotten into a relationship with a late 40s woman.